The invention relates to a method and apparatus for improving the fatigue strength in weld zones of any of welded structures.
The technique of forming one complete article by bonding two or more parts through welding is employed for the manufacture of various products in substantially all of the technical fields. However, the welding parts of large dimension requires a number of steps, and therefore, various disadvantages are brought about. More specifically, according to an ordinary welding method, a weld metal is deposited and embedded or fused in portions of the two parts that are to be bonded, and the portions are bonded to each other by the deposited weld metal. Accordingly, in order to faciltate embedding of the weld metal in the portions to be bonded and impart a sufficient strength to the bonded portions with respect to large pieces of metal, it is necessary to shape the exposed area of the adjacent faces so that the weld metal may reach the interior of the adjacent surfaces or faces. This is conventionally accomplished by forming a groove that is V-shaped or U-shaped at the exposed area of the adjacent surfaces to be bonded to assist in depositing the weld metal to the interior of the adjacent surfaces. In general, the embedding or penetration of the weld metal into such a V-shaped or U-shaped groove is accomplished by melting a welding rod at the bottom of the groove. When the parts to be bonded have very large dimensions, the size of the V-shaped or U-shaped groove into which the weld metal is embedded and penetrated, such naturally be increased according to the conventional method and the embedding of the weld metal by using the welding rod should be conducted repeatedly so as to build up successive layers of deposited weld metal in order to fill such a large dimensioned groove. Therefore, many steps are required for the formation of such a weld joint employing a V-shaped or U-shaped groove for large dimensioned materials, wherein weld metal is embedded and penetrated.
If the size of such V-shaped or U-shaped groove for the embedding of a weld metal is diminished beyond what is necessary to accomplish complete penetration of the weld metal into the clearance between the adjacent surfaces, it is then possible to reduce the number of steps for welding the parts to be bonded and further to reduce the frequency of repeating the metal depositing that would be required to fill up the groove through successive deposits. However, the strength of the weld would correspondingly decrease.
In short, according to conventional welding methods, a groove is formed in the welding zone between the parts to be bonded, and with large dimensioned parts there are formed areas in which weld metal is deposited and areas in which weld metal is not deposited between the adjacent surfaces of the two parts to be joined.
In such welded structure, the non-deposited portions in the finished product behave as if they were cracks in metal parts, and when an external force is imposed in the bonded portions, the stress is concentrated at the top ends of such cracks, which will have a notch effect at the roots of the non-deposited portions, and the fracture is accelerated in the weld zone, particularly with respect to fatigue strength. Therefore, the fatigue strength is very low in such a structure.
As a means for improving the fatigue strength in the weld zone of such welded structures, there is known a method in which an external force greater than the external force to be imposed on the welded structure at the actual application is imposed on the weld zone temporarily in advance to actual use of the structure.
More specifically, an external force is imposed on a metal having cracks in such a direction that the cracks are enhanced, whereby a stress concentration on the end portions of the cracks is caused to form a zone locally yielded due to reaching the yielding stress. When application of the external force is stopped in this state, a zone reversely yielded is formed in the end portions of the cracks. Namely, when a tensile force is applied as the external force enhancing the cracks, a tensile-yielding zone is formed in the end portions of the cracks, but when this external force is removed, a compression-yielded zone is formed because of the restraint from the remaining unyielded portions of the metal.
After such treatment when a concentrated stress by an external force is imposed during actual operations and is smaller than the concentrated stress by the above-mentioned preliminary applied external force, an action of confining the crack is manifested by the compression-yielded zone and as a result, the fatigue strength is improved. As described above, it is known that the fatigue strength can be improved if an external force greater than the external force imposed on a welded structure at the actual application when in use is preliminarily imposed to the unwelded portion of a weld zone. However since it is difficult to apply an appropriate external force to a weld zone having an unwelded portion, this known method is not usually practical and is hardly ever employed.
For example, it is considered that a necessary external force may be obtained by attaching a welded part having an unwelded portion to a rotary shaft, rotating the part together with the shaft utilizing a centrifugal force generated by the rotation. According to this method, however, it seldom happens that the external force is effectively imposed only on the unwelded portion, but an excessive external force is supplied even to an unnecessary portion, that is a portion that need not be yielded and is outside of the stress concentration zone so that the part will become damaged. Accordingly, it is very difficult to adjust the external force to be applied to the unwelded portion while controlling appropriately the external force to be applied to a portion outside of the stress concentration zone.
It has also been considered that an external force may be applied by using an oil pressure jack. But according to this method, it is difficult to apply a necessary external force to the appropriate portion. Further, since an external force greater than an external force to be imposed when the product is actually put into use, must be employed for this preliminary treatment, it is necessary to take great care not to damage the welded portion.